I'm unable to find the button on my homepage of Mobile Security (Ice Cream Sandwich) that allows me to block texts/calls. This was a selling point for me.
The only four icons I see are Anti Melware, Anti Theft, Backup, and Web Protection.
It appears that you are not the only one. You may like to look at this thread started by JohnP51 which shows you are not alone, though there do not seem to be many suffering this problem.
Can you tell us what device you are using, what version of Android and what version of Norton Mobile Security, please. That might help the team sort the problem out sooner. I know they are working on it. ;-)